[CMake] SET_SOURCE_FILES_PROPERTIES GENERATED not working?

[Reggie Burnett]

Today we suddenly have a situation where a file that is generated by a tool
during building is being looked for by cmake.  We have a
SET_SOURCE_FILES_PROPERTIES call to set it to GENERATED but we still get an
error that the file is not found during cmake process.  
 
Are there any special tricks or gotchas related to this?
 
Thanks
Reggie